Prevention of casing corrosion



vSept. 25, 1956 G.. H. ROHRBACK ETAL 2,764,242

' PREVENTION OF CASING CORROSION Filed April 29. 1953 2 Sheds-Sheet 1 lS RFACE CA ING AIR CEMENT DRILLI G M D COR OSIOI :HIGH PRESSURE SALTWATER PRODUCTION TUBING L CASING Flew INVENTORS G/LSON' H ROHRBACKJOSEPH F CH/TTUM BY M ATTOR YS P 1956 G. H. ROHRBACK ET AL 2,764,242

PREVENTION OF CASING CORROSION Filed April 2-9, 1955 2 Sheets-Sheet 2 BB 4 \\\\U I 2 l I 6 I! A A FIG.2

INVENTORS G/LSON H. ROHRBACK United States Patent PREVENTION or CASINGcoRRosmN Gilson Rolrrback, Seattle, Wash, and Joseph F.

Chittum, Whittier, Calif.

Application April 29, 1953, Serial No. 351,831

4 Claims. or. 166-1) This invention relates'to a composition and methodfor inhibiting the corrosion of oil well casings.

Although relatively little attention has been given to corrosion failureof oil well casings in the literature, this problem is ratherwide-spread. A number of theories have been proposed to account forcasing corrosion and some attempts to control it have been made on thebasis of the theories. Among the theories heretofore advanced to explaincasing corrosion have been suggestions that the corrosion was due tobacterial action; due to electric currents flowing through thecasing;due to localized corrosion at metal flaws; due to particularcompositions of formation water; due to corrosion by selfpotentialcurrents, and due to interzonal migration of salt water. So far as isknown, no successful method of controlling casing corrosion has beendeveloped on the basis of any of these theories. Meantime, casingfailure due to corrosion continues to be a serious problem in many oilfields. Repair of casing failure is difficult at best and if the failureescapes detection for an appreciable period of time, the well ceases toproduce and production may not be resumed, even though the casing berepaired, necessitating abandonment of the well.

Thirty instances of casing failure due to corrosion in I a Californiafield were carefully studied. The average time of failure was about fiveyears, but failures occurred at times ranging from less than one year toabout sixteen years from completion of the well. In the large majorityof these cases the failure occurred in the salt water zone above thecemented zone. Corrosion of the casing in all instances was highlylocalized and from an overall study of the circumstances attending thesecasing failures it was concluded that the-corrosion was electrochemicalin character and occurred when a part of the casing was in contact withlow (7-8) pI-l formation waterand a part of the casing was in contactwith higher "(8 to 12) pH mud or with higher (8-12) pH washingsolutions. The situation existing in the well when serious .corrosionisencountered will be better understood by reference to Fig. l of theappended drawings, which is a diagrammatic illustrationof a section of atypical well casing and formation. The portion of the casing in contactwith formation water ofrelatively low pH is the area at which rapidcorrosive attack appears to occur.

It has now been found that casing corrosion can be very markedlyinhibited by filling the annular space above the cemented zone andbetween the casing and the formation with an aqueous clay drilling mudcontaining .001 .to 2.5%- by weight of ferrous chloride or stannouschloride. When a well has been drilled to the desired depth and is readyfor a cementing operation, the casing is full of drilling mud. Cementingof the casing is commonly accomplished by forcing a cement slurry(optionally preice of cement slurry has been introduced into the casing,the cement is forced into position between the outer casing wall and theformation by pushing it down the casing with additional mud. When thecement is in place it is allowed to set and that portion of the annulusbetween the outer casing wall and the formation and above the cementedzone is filled with drilling mud which was pushed ahead of the cementslurry. This drilling mud should have a substantial content of eitherferrous chloride or stannous chloride in order to prevent casingcorrosion. Similarly, it is desirable that the mud introduced into thecasing to force the cement into position and which is ultimatelyresident in the annulus between the casing and the formation below thecemented zone should also be treated so as to have an appreciablecontent of either ferrous chloride or stannous chloride.

While ferrous chloride and stannous chloride are the preferred ferrousand stannous compounds for use pursuant to the invention, it is theferrous and stannous ions which are the effective inhibiting additives.Accordingly, other ferrous and stannous compounds which are soluble ordispersible in aqueous fluids such as the sulfates, the nitrates, thehydroxides, can be used instead of the chlorides in equivalent amountsand are found essentially equally effective.

During the drilling of a well the mud is continually circulating and iscontinually in contact with the air. No advantage is obtained by addingferrous compounds or stannous compounds to the mud used during thedrilling, since the regular and intimate contact with the air in the mudpits would cause oxidation of stannous and ferrous compounds to thecorresponding stannic and ferric compounds which are useless inpreventing corrosion. Accordingly, the stannous compound or ferrouscompound must be added to that portion of the mud which will be leftbetween the outer casing wall and the formation when the well iscompleted just prior to introducing this portion of the mud into thewell. Both stannous chloride and ferrous chloride, especially thelatter, increase the viscosity of aqueous clay muds and increase theirrate of filter loss.

It has been found that corrosion can be inhibited without adverse effecton the viscosity and water retaining properties of the mud by adding acorrosion inhibiting concentrate consisting essentially of a mixture of4 to 20 parts by weight of stannous chloride or ferrous chloride and 2to 10 parts by weight of a water loss reducing agent to that portion ofthe mud which will ultimately be resident between the casing wall andthe formation immediately prior to the introduction of that mud into thewell. Suitable water loss reducing agents are starch, natural gums, suchas gum arabic, gum tragacanth, Indian gum, and the like, or the alkalimetal salts of carboxymethylcellulose. The carboxymethylcellulosematerials are especially suitable and mixtures of ferrous chloride andthe carboxymethylcellulose salts containing between 1 and 10 parts byweight of ferrous chloride per part by weight of thecarboxymethylcellulose compound constitute especially suitable corrosioninhibiting concentrates.

It is frequently desirable to employ a corrosion inhibiting concentratecontaining a viscosity reducing agent in addition to the ferrouschloride and water loss reducing agent. For example, suitableconcentrates will contain from 4 to 20 parts by weight of ferrouschloride, 2 to 10 parts by weight of a water loss reducing agent, and 3to 30 parts by weight of a viscosity reducing agent. Mud viscosityreducing agents are well known to those skilled in the art and includethe molecularly dehydrated phosphates or polyphosphates and planttannins such as quebracho extract and chestnut extract. An especiallydesirable concentrate is one containing 4 to 20 parts by weight offerrous chloride, 2 to 10 parts by weight of an alkali metal salt ofcarboxymethylcellulose, 3 to 12 parts by weight of a sodiumpolyphosphate, such as sodium meta-phosphate, sodium tetraphosphate,sodium tripoly- 4 by the fluid of lower pH. The electrodes were thenleft in these relative positions for a period ordinarily of 14 days, atthe end of which electrode 4 was removed and weighed to determine itsloss of weight. In order to dephosphate, tetrasodium pyrophosphate,sodium hexa-rneta- 5 termine the weight loss of electrode 4 due toelectrophosphate, and the like, and 4 to 20 parts of quebracho. chemicalaction alone, a second small iron-strip approxi- The inhibitorconcentrates described above are added mately i ntical with electrode 4in Size andshape Wa to the mud amount sufficient to give the mud aferrous suspended from the Wall 0f the i in Same fluid chloride contentin the range from 0.001% to 2.5% by whiehsllffellllded electrode the endof the test weight. Compositions of typical d treated d o period theweight loss of electrode 4 and the weight loss treated are indicatedbelow. The untreated mud had a of the Second metal p. e determined andthe density of 90 pounds per cubic foot and a pH of 11 terence betweenthese two losses was the loss of weight of electrode 4 due toelectrochemical action. Results Untreated Treated of a series ofexperiments are set forth in Table I be- Po u ds P05123 15 low. The mudsemployed in the tests were typical com- 33 33 2 3 mercial drilling muds.The formation water was a typical of Mud 0f Mud aqueous effluent from aCalifornia well and the .pyrofluid was an aqueous solution oftetrasodium pyrophosphate gigg %.288 such as is commonly employed forwashing the mud gg gfi 'flfigflhigg' jj" Nono cake prior to introductionof the cement and which is $222 gf lf l gg commonly left in the annulusbetween the outer casing quebrachounir p wall and the formation when thewell is completed. The Sodium Hydmxide 150 150 efiect of variousadditives on the weight loss due to electrochemical corrosion is shownin the table.

TABLE I Weight loss in corrosion cells for difierent combinations oftreated and untreated fluids Electrode 1 Electrode 4 1\{easurcd VcightTest No.

Fluid Additive Loss from Electrode 4 Due to Ourrent (Milllgrems) pHFluid Additive pH N one Quebracho Sodium Ohrornate.-.

. Formation Water- 2-Mud contained quebracho-OAZ percent.

Laboratory studies of the nature of casing corrosion and of compositionsand methods for controlling it were made in the apparatusdiagrammatically illustrated in Fig. 2 of the drawings.

In the drawing container 6 is a glass jar having about 4 gallonscapacity. Electrode 1 Was made up of three short sections of concentricpipe welded together. The weld areas were covered with a plastic paintto eliminate undesirable galvanic couples from these areas. The totalarea of this electrode was about 330 square inches. Electrode 4 is asmall strip of iron having a surface area of approximately 2 squareinches. Electrode 1 is welded to iron rod 2 which is attached toadjustable support 5 so that the position of electrode 1 in the jar canbe adjusted at will. Electrode 4 is connected to rod 2 by wire 3. Inmaking the experimental tests to fluids were introduced into jar 6. Thefluids differed from each other in both pH and density. In each test,jar 6 was positioned in the jar so that it was entirely surroundedpletely eliminated by the addition of eitherferrous chloride or stannouschloride to the high pH fluid, which in practical effect means theaddition of these materials to the mud and to the pyrowash solution ifsuch a solution is used in completing the well.

l. A corrosion inhibiting concentrate, suitable for addition to aqueousclay drilling mudto be left in the anfilled with the denser of the twofluids employed to level nular space between the casing and theformation of a AA and then the less dense of the two fluids wasintrocompleted oil well, consisting essentially of 4 to 20 parts -ducedinto jar 6, filling the jar to about level BB. Elecby weight of amaterial selected from the group controde 1 was positioned in the jar sothat it wasentirely sisting of stannous chloride and ferrous chlorideand 2 surrounded by the fluid of higher pH. Electrode 4 was r to 10parts by weight of a water loss reducing agent.

2. A corrosion inhibiting concentrate suitable for addition to aqueousclay drilling mud to be left in the annular space between the casing andformation of a completed oil Well consisting essentially of ferrouschloride and an alkali metal salt of carboxymethylcellulose containing 1to 10 parts by weight of ferrous chloride to each part by weight ofcarboxyrnethylcellulose.

3. A corrosion inhibiting concentrate, suitable for addition to aqueousclay drilling mud to be left in the annular space between the casingandthe formation of a 1 completed oil well, consisting essentially of 4 to20 parts byweight of ferrous chloride, 2 to 10 parts by weight 'of analkali metal salt of carboxymethylcellulose, 3 to 12 parts by Weight ofan alkali metal polyphosphate and 4 to 20 parts by Weight of quebraeho.

4. The method of preventing external corrosion of oil well casing whichcomprises filling the annular space above the cemented zone and betweenthe casing and the formation with an aqueous clay drilling mudcontaining 0.001 to 2.5% by weight of a material selected from the groupconsisting of ferrous chloride and Stannous chloride.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Hoar et al.: Data on the Use of Stannous Chloride as aPickling Inhibitor, article in Chemical Abstracts, vol. 35, 3936, 1941.

1. A CORROSION INHIBITING CONCENTRATE, SUITABLE FOR ADDITION TO AQUEOUSCLAY DRILLING MUD TO BE LEFT IN THE ANNULAR SPACE BETWEEN THE CASING ANDTHE FORMATION OF A COMPLETED OIL WELL, CONSISTING ESSENTIALLY OF 4 TO 20PARTS BY WEIGHT OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OFSTANNOUS CHLORIDE AND FERROUS CHLORIDE AND 2 TO 10 PARTS BY WEIGHT OF AWATER LOSS REDUCING AGENT.